mpeg2 to vcd : does input resolution have no effect on output quality ?!

[Koorosh]

i converted two mpeg2 files to vcd format using TMPGEnc.
here are input files resolutions and bitrates :
---------------------------------
first :
704 X 576
average bitrate : 3745 kb/s
---------------------------------
second :
352 X 288
average bitrate : 1132 kb/s
---------------------------------
the quality of the first file was obviously better than the second so i expected the larger resolution to have better quality when converted.
but the quality of the output files is the same ( about VHS quality ).
it seems input resolution ( and bitrate ) is not very important when doing mpeg2 ----> vcd.
i say "it seems" coz i'm not sure about that .
is the resolution realy no important (or less important) when encoding ?

[Nick]

VCD format has specifications for resolution and bitrate.
352x288 for PAL, with a fixed, constant bitrate of 1150Kb/s.

So when you say you converted your high-res file to VCD format, what you actually did is to resize and re-encode it to make the resolution and bitrate very similar to your second file!

Unsurprisingly, therefore, the results are rather similar.
If your player supports SVCD, this may be a more suitable option for the higher resolution file.

[Koorosh]

Originally posted by Nick
VCD format has specifications for resolution and bitrate.
352x288 for PAL, with a fixed, constant bitrate of 1150Kb/s.

So when you say you converted your high-res file to VCD format, what you actually did is to resize and re-encode it to make the resolution and bitrate very similar to your second file!

Unsurprisingly, therefore, the results are rather similar.
If your player supports SVCD, this may be a more suitable option for the higher resolution file.

thanks Nick

but the second file has been encoded too(resulting loss of quality).
then you are saying that the source resolution and bitrate has no effect on output quality ?
excuse me but i'm a bit confusing about that.

[Nick]

I'm certainly saying there is no advantage to having a source which is higher than the target resolution. Your VCD is stuck with being 352x288. So any source of higher resolution has to be resized down to that resolution anyway.

As for source bitrate, the higher the quality of your source, the better chance of a good result after conversion. However, VCD is MPEG1, and neither MPEG1 nor MPEG2 are lossless formats. So you are encoding from one lossy compressed format (MPEG2) to another. This inevitably involves deterioration of quality.

[Koorosh]

Originally posted by Nick
I'm certainly saying there is no advantage to having a source which is higher than the target resolution. Your VCD is stuck with being 352x288. So any source of higher resolution has to be resized down to that resolution anyway.

As for source bitrate, the higher the quality of your source, the better chance of a good result after conversion. However, VCD is MPEG1, and neither MPEG1 nor MPEG2 are lossless formats. So you are encoding from one lossy compressed format (MPEG2) to another. This inevitably involves deterioration of quality.

thank you very much Nick for your reasonable description.

well ... i have another question (hopefully my last) :
is the story valid for all lossy formats ?

[Trahald]

Originally posted by Koorosh
thank you very much Nick for your reasonable description.

well ... i have another question (hopefully my last) :
is the story valid for all lossy formats ?

By definition, yes. The decoded destination will not be equal to the decoded source. There could be a lossy format that says after the bitrate is too high to do anything anymore that it will save the original pictures, but then its more of a hybrid and not fully lossy.

[Koorosh]

Originally posted by Trahald
By definition, yes. The decoded destination will not be equal to the decoded source. There could be a lossy format that says after the bitrate is too high to do anything anymore that it will save the original pictures, but then its more of a hybrid and not fully lossy.

thanks a lot moderator :)

at last someone put this subject to rest for me. :D